1. Field of the Invention
The present invention relates to a liquid-phase growth method and a liquid-phase growth apparatus for growing a semiconductor layer on an oxygen-containing substrate and relates to a solar cell.
2. Related Background Art
Recently, a solar cell has widely been employed for an independent power source to drive various kinds of electronic appliances and apparatuses and also for a power source systematically interrelated with commercial power source. Generally, silicon and gallium arsenide are used for a semiconductor constituting a solar cell. In order to obtain high photoelectric conversion efficiency (efficiency of the conversion of light energy into electric energy), their single crystals or polycrystals are preferable.
It has also recently been recognized that use of an epitaxial silicon substrate is extremely preferable to fabricate a bipolar LSI and a CMOS-LSI. First of all, independent control of impurity concentration in a bulk part and an epitaxial layer (an active layer) can easily be carried out in an epitaxial substrate. Respective transistors of a bipolar LSI can easily be shut from one another and electric interference among respective transistors can be prevented by utilizing such a characteristic. The latch up phenomenon of a CMOS-LSI can be prevented, too.
Further, since an epitaxial substrate scarcely has as-grown defects, which are sometimes observed in a conventionally used silicon substrate grown by the Czochralski process, a device with improved dielectric strength of an oxide film and with high reliability can be fabricated in the case of a CMOS-LSI. Also, since heavy metal impurity removal is remarkably efficiently carried out for an epitaxial substrate using a substrate containing boron (B) in a high concentration by gettering treatment, a device fabricated using the epitaxial substrate has been known to be generally provided with excellent performance characteristics.
Moreover, in the case of application to a solar cell and an epitaxial substrate, a silicon layer with 10 xcexcm to 100 xcexcm thickness is often required and it is possible to provide an epitaxial layer with such a thickness by a CVD method. However, the number of substrates to be loaded per a batch is limited or a maintenance work of the apparatus to be employed is required after every batch and the production throughput is thus low.
In order to heighten the production throughput, silicon layer growth by a liquid-phase growth is advantageous. Detailed description of solar cell fabrication by liquid-phase growth is described in Japanese Patent Application Laid-Open No. 10-189924. The technique is basically applicable not only to a solar cell but also to production of an epitaxial substrate, but in the present state, it still leaves much room for improvement in the defect density of a grown silicon layer and the life time of minority carriers.
By a conventional technique, a crystal is grown on a substrate by melting a metal with a relatively low melting point such as indium, gallium, tin, aluminum, copper, or the like, dissolving a semiconductor raw material such as silicon or the like in the melted metal to form a solvent (hereafter called as xe2x80x9cmeltxe2x80x9d), immersing a substrate in the melt, and precipitating the oversaturated semiconductor raw material on the substrate. In order to grow a thick film at a high speed, it is necessary to previously dissolve a semiconductor raw material as much as possible in the melt.
Generally, the solubility of a solid in a liquid becomes higher as the temperature is higher. For that, in the case indium having a low melting point and capable of growing a silicon crystal with high quality is selected among the metals usable as a melt, it is difficult to carry out crystal growth at a rate of 0.1 xcexcm/minute or more when a melt temperature is lower than 750xc2x0 C., thereby sometimes resulting in hindrance to production of a solar cell and an epitaxial substrate required to have a thick epitaxial layer. In order to heighten the throughput for mass production, it is desirable that the temperature of the melt is set to, e.g., 850 to 1,050xc2x0 C.
A silicon wafer produced by the Czochralski (Cz) process is widely used for producing an epitaxial substrate. Since such a Cz silicon wafer is produced by heating silicon in a crucible made of quartz glass at a high temperature not lower than 1,400xc2x0 C. to melt silicon, the silicon wafer inevitably contains excess oxygen.
FIG. 7A is a cross-sectional view showing a substrate 200 containing oxygen 201. Like that, oxygen 201 is consequently contained in the substrate 200 and owing to that, the resultant silicon crystal is provided with an advantage that viscosity is improved to prevent damage on the substrate during the production process. Further, oxygen 201 is said to work as a gettering sink and to lower bad effects of contamination with heavy metal atoms and there are other advantage such as low cost.
However, regarding the Cz wafer, the quality of the grown epitaxial film on the Cz wafer is sometimes deteriorated due to the contained oxygen. FIG. 8 shows microscopic photographs of the surfaces of a Cz silicon substrate with (111) plane orientation taken after heating the substrate for a predetermined time and then chemically etching the resultant substrate (cited from Fumio Shimura: xe2x80x9cSemiconductor Silicon Crystal Engineeringxe2x80x9d, published by Maruzen, FIG. 6.58). As expected from FIG. 8, as the temperature of the melt increases, the density of actual defects is increased and at 750xc2x0 C. or higher, the density is sharply increased.
This is, as shown in FIG. 7B, because oxygen 201 contained in the substrate 200 is precipitated and appears as defects 202 by treating the substrate 200, for example, in a melt at 750xc2x0 C. or higher. The defects 202 appearing on the surface of the substrate 200 can be cores of stacking faults 204 caused in a semiconductor layer 203, so that the epitaxial layer formed thereon possibly contain a large number of stacking faults 204 (FIG. 7C).
A stacking fault 204 is known to deteriorate various characteristics of a semiconductor device and highly harmful for application to a solar cell as well as for application to an epitaxial substrate. Thus, there is a problem that if the temperature of the melt is increased to obtain a practical growth rate of an epitaxial film, the quality of the epitaxial layer has to be sacrificed.
Further, an epitaxial growth apparatus to be employed at a high temperature such as a CVD apparatus and a liquid-phase growth apparatus generally comprises main parts such as a reaction tube, a crucible, or the like made of quartz glass, and however these members contain heavy metals in a small amount. The heavy metals in constituent members of an electric furnace are said to permeate the tubular wall of a reaction tube at a high temperature.
Moreover, metallic materials such as a stainless steel are inevitably to be used for members such as pipes and valves to which heavy load is applied in a large scale apparatus for mass production. For that, during the period from the time when a substrate is loaded in an apparatus to the time of starting epitaxial growth, the surface of the substrate may be polluted with heavy metal atoms. Furthermore, heavy metal atoms such as iron, chromium, copper or the like are agglomerated with the lapse of time and defects 403 of such as silicides are caused in the surface of the substrate 200 (FIG. 9C). That is illustrated in FIG. 9A to FIG. 9C.
In the case epitaxial growth is carried out on the substrate 200 polluted with the heavy metal atoms 401, 402, the defects 403 become cores and may cause stacking faults 405 in the resultant epitaxial layer 404 or the heavy metal atoms 406 may be diffused in the epitaxial layer 404 (FIG. 9D). The stacking faults 405 and the diffused heavy metal atoms 406 considerably deteriorate the quality of the epitaxial layer 404.
It is an object of the present invention to grow a semiconductor layer with extremely few defects even if a substrate contains oxygen.
It is another object of the present invention to grow a semiconductor layer with extremely few defects even if the substrate surface is polluted.
According to the present invention, there is provided a method for producing a semiconductor substrate, which comprises the steps of growing a first silicon layer on a silicon substrate in liquid phase at a temperature lower than 750xc2x0 C. and growing a second silicon layer on the first silicon layer in liquid phase at a temperature not lower than 750xc2x0 C.
According to the present invention, there is provided a solar cell produced by a method comprising a step of anodizing a surface of the first and second silicon layer side of the above-described semiconductor substrate.
According to the present invention, there is a liquid-phase growth method for growing a semiconductor layer on a substrate containing oxygen, which comprises growing a first semiconductor layer by bringing the substrate into contact with a melt at a temperature such that defects due to the oxygen are suppressed to be less than 1000/cm2 on a surface of the substrate.
According to the present invention, there is provided a liquid-phase growth apparatus for producing the above-described semiconductor substrate, comprising means for storing a melt in which silicon is melted, means for changing a temperature of the melt, and a holding member for simultaneously holding a semiconductor material to be melted in the melt and the silicon substrate; and means for transporting the holding member up and down.
In order to achieve the foregoing purposes, in the liquid-phase growth method of the present invention for growing a semiconductor layer on an oxygen-containing substrate, a first semiconductor layer is grown by bringing the substrate into contact with a melt at a temperature such that a number of defects derived from oxygen in a surface of the substrate is suppressed to be less than 1000/cm2.
A solar cell of the present invention is provided with the first semiconductor layer formed by the above-described liquid-phase growth method.
The method of the present invention for producing an epitaxial substrate comprises using the first semiconductor layer formed by the above-described liquid-phase growth method as an epitaxial layer. Further, the method of the present invention for producing the epitaxial substrate comprises growing a second epitaxial layer at a high speed by bringing the substrate having the first semiconductor layer formed thereon into contact with a melt at a temperature higher than that for formation of the first semiconductor layer.
Moreover, a liquid-phase growth apparatus of the present invention comprises means for storing a melt, means for changing the temperature of the stored melt, and means for bringing an oxygen-containing substrate into contact with the melt, wherein a first semiconductor layer is grown on the substrate by bringing the substrate into contact with a melt at a temperature such that a number of stacking faults contained in the semiconductor layer on the surface of a silicon substrate is suppressed to be less than 1,000/cm2.